Hello~You said you have been to two doctors, have you been to a cardiologist? I really think you should ask for an echocardiogram, stress test and an event monitor that is worn for either 24 hours or an entire week. If the pain is related to your heart, then it will surely show up in at least one of these tests.
If all these test have good results, then I would suggest seeing a chiropractor, you may have some pinched nerves in your mid to upper back causing the pain.